This invention relates to an operating arrangement for a gaseous fueled engine, and more particularly to an improved apparatus and method for operating an engine on liquid petroleum gas (gaseous fuel) to provide good emission control and uniform running under a wide variety of conditions and during a long period without necessitating major servicing.
In the interest of conserving natural resources and still further reducing the emission of unwanted constituents to the atmosphere from the exhaust of internal combustion engines, renewed interest is being shown in the operation of the engines on other forms of fuel than gasoline. The advantages of operation on gaseous fuels (LPG) is again being investigated as a possible source of achieving these goals.
In one successful arrangement for operating an engine on gaseous fuels, the engine is supplied with a charge former, which mixes the gaseous fuel with the atmospheric air and supplies it to the engine. The charge former has one or more control circuits, specific examples of which will be described later, for providing the desired fuel-air ratio in response to the engine running conditions. In addition, a feedback control system is employed using an exhaust sensor so as to ensure that the engine is always operating at a stoichiometric condition, sometimes referred to as .lambda.=1. The value of .lambda. is arrived at by taking the actual fuel-air ratio at which the engine is running and dividing it by the ideal stoichiometric ratio at the same running condition. Frequently, an oxygen sensor is employed in the exhaust system for providing this feedback control.
The output of the exhaust sensor is transmitted through a CPU to a device for making fine adjustments in the fuel-air ratio. One way in which this is done is by bleeding air into the source of gaseous fuel supplied to the circuits of the charge former.
The value of using a so-called "air valve" carburetor in conjunction with such engines has also been acknowledged. Such carburetors employ a throttle valve in the induction system, which moves to a position to maintain the desired pressure ratio across the throttle valve. The throttle valve is then coupled by some means such as a linkage system or the like to a metering circuit of the fuel supply so that the fuel-air ratio is maintained appropriately for the engine running condition.
As noted above, superimposed upon this charge-forming system is a feedback control system that operates a bleed control valve for sufficient air into the fuel circuits to maintain the .lambda.1 condition. Art example of a system operating on this principle is shown in the United States Letters Patent entitled "Fuel Control and Feed System for Gas-Fueled Engine," U.S. Pat. No. 5,337,772 issued on Aug. 16, 1994, which patent is assigned to the Assignee hereof.
In connection with the use of the feedback control, the air bleed control valve is of a type that is operated by a stepping motor. The position of the stepper control is correlated by the electronic control to maintain the desired .lambda.1 condition. In order to assure a good range of adjustment, the initial setting of the air bleed valve is normally chosen for a given steady-state condition to be one-half of its total range of operation. For example, if the stepper motor moves through 100 steps from a fully closed position where there is no air bleed and fuel feel and a fully opened position where there is maximum air bleed and fuel dilution in 100 steps, the initial stepping will be at the 50-step position, with adjustments being made is either to maintain the necessary .lambda.1 condition.
However, even though there is an automatic feedback control, the condition of the various components may change with either time or for other reasons such as deterioration of certain components in the system due to carbon deposit, etc. The feedback control system will, of course, maintain the .lambda.1 condition, but the initial setting of the stepper motor may thus necessitate deviation from the standard 50-step position to another position such as, for example, a 70-step position. Therefore, the range of leaning the mixture will be decreased when this situation occurs because there will only remain 30 steps for leaning. Of course, the drift may occur in the other direction, and thus limit the amount of enriching that is possible.
It is, therefore, a principal object of this invention to provide an improved operating arrangement and fuel control system for a gaseous fueled engine wherein an arrangement is provided for permitting adjustment of the air bleed to accommodate for changed conditions in the engine and its components without necessitating a change in the position of the computer-controlled air bleed circuit.
It is a further object of this invention to provide an improved and simplified arrangement for a gaseous fueled engine that can accommodate for drift from normal condition due to wear, failures, etc.
As has been previously noted, various types of sensors may be employed in the exhaust system to provide the necessary signal to provide the required feedback control. The most common sensor employed is an oxygen (O.sub.2) sensor. Although O.sub.2 sensors are extremely effective, they do have some disadvantages. For example, the normal oxygen sensor only outputs a signal when the .lambda.1 condition is exceeded. In addition, the oxygen sensors also do not provide a signal until they reach an operating temperature, normally something in the range of 350.degree. C. As a result, the electronic control or computer must be programmed so as to afford a control for the fuel-air ratio under conditions when the sensor is not outputting a signal either because it is not at an operating temperature, because it is not possible to determine whether it is not at an operating temperature or the engine is running lean, or because the sensor may actually fail.
It is, therefore, a still further object of this invention to provide an improved operating arrangement for a gaseous fueled engine wherein a control routine is established for conditions when feedback control is not possible.